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Related: backup, RAID, RAID-0, RAID-1, RAID-4, RAID-5, storage
RAID can be implemented in hardware or software.
Most computer users do not use RAID at all, using instead independent hard drive or SSD storage. That approach is least expensive and fine for most uses.
RAID is not for backup
It’s not that RAID (e.g., a mirror) is a bad idea for backup, rather, it is generally a waste of money and a reduction in redundancy: it is always better to have multiple independent redundant backups than one RAID backup.
For example, two independent backup drives of 4TB each are better than one RAID-1 mirror single unit. In the dual case, failure of one unit leaves the other intact and they can be stored separately, further reducing risk; in the single-unit RAID-1 case, loss of the unit means all is lost (consider theft, fire, flood, etc).
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Why use RAID?
For professionals, higher performance and/or fault tolerance might be helpful or even essential, and that is where RAID comes into the picture.
|RAID-5||RAID 1+0||RAID-0 stripe||RAID-1 mirror||RAID SPAN||JBOD|
|# Drives||4||4||4||4||4, concatenated||4
|Fault tolerance:||single failure OK||single failure OK
from each mirror
|single failure kill volume||three failures OK||single failure kills volume||independent|
|Read Speed||~3X||2X||4X||1X||1X, variable||1X|
|Write Speed||2.2X - 3X||2X||4X||1X||1X, variable||1X|
Relative performance of RAID- vs RAID-5